The common name grunion most often refers to the species Leuresthes tenuis, or more appropriately the California grunion*. It is a fantastic saltwater fish native to the Pacific ocean near California. This rather ordinary looking fish has gained international renown for its unique reproductive behaviour: it is one of the only fish species to come ashore to spawn (mudskippers would be the other that I can think of offhand). The name grunion is taken from the colloquial name the Spaniards gave the fish grunion, meaning 'grunter'.

General Biology and Behaviour

The grunion is a small, slender fish with an average length of 5 inches (12 centimetres). They are blue-green dorsally and silver ventrally and laterally. Being members of the Atherinidae, the silversides, they have large pectoral fins, are excellent swimmers and lack an adipose fin.

The California grunion can be found from point Conception in southern California to Punta Abreojos in Baja, Mexico. There are some small populations north and south of these boundaries, and spawning has been reported as far north as Monterey Bay.

Grunions have a very rapid growth rate, and can grow to be 5 inches long in their first year of life. They are sexually mature at one year of age, and only live two to three years in most cases. After the fish begin to spawn, their growth rate slows remarkably as much of their energy is devote to the production of eggs and sperm (that is, they produce gonadal tissue rather than somatic tissues). In the past, biologists would have called this species an r-selected species (this classification has recently fallen out of favour).

Surprisingly little is known about the behaviour of the grunion when it is at sea. They apparently live close to shore in water roughly 5-10 meters deep, and are obligate planktivores. A great deal is known about their spawning behaviour, on the other hand.

Grunion begin to spawn in late February or early March, and continue throughout the spring and summer. Normally, the spawning cycle ends in August or September. The number of weeks that the species spawns varies every year. The grunion spawn at night, and only for a few hours on the three or four nights associated with the full moon or new moon and the high tides that accompany them (more on this later). The spawning run normally begins with a single fish riding a wave onto the shore, and often stranding itself on the sand. More and more fish come in with the waves and continue to strand themselves ashore. About 20 minutes later the beach is covered with writhing fish, and spawning begins. An hour after the first fish strands itself, spawning reaches its peak. About an hour later, when the tide has dropped a foot or more, the fish work their way back into the water and no more fish will be seen until the following evening.

During the spawning period, females will ride a wave onshore accompanied by as many as eight males. If she can't find any males on the beach, she will head back to the ocean. If there are males around her, she will work her way as far up the beach as possible and thrust her body into the sand. She then arches her back and waves her tail back and forth, working it into the sand. She finishes by aligning herself vertically, with her entire body up to the pectoral fins buried in the sand. The female will then continue to twitch and writhe, releasing her eggs into the sand at about 2-3 centimetres depth. Larger females may lay up to 3000 eggs at a time. During this time, males will surround her and discharge their sperm onto the sand. Their sperm flows down the body of the female, and thus fertilization takes place under the surface. The males then flee back to the ocean, leaving the female to extricate herself and make her way back to the water. All of this takes place in about 30 seconds.

The fertilized eggs are further buried by the action of the outgoing tides, and find themselves 8 to 16 inches deep by the time the tide retreats. After about 10 days, the eggs are viable and ready to hatch, but the remain inactive until they are freed by the next series of high tides. A mere two to three minutes after the eggs are exposed, they hatch and the larvae make their way to the open ocean.

The grunion's reproductive cycle has evolved to be highly synchronized with the action of the tides. They will come to shore to spawn when the tides are just past their peak intensity, as the full moon wanes towards the first quarter or as the new moon moves to the third quarter. Further, their eggs have evolved the ability to lay dormant waiting for the next high tide event, although they are developed and viable when the tides are at their lowest. During the spawning period, the relation of the species to the moon is as follows:

  • Full moon -- eggs from the previous spawning run will be freed and will hatch
  • Between full moon and first quarter -- Grunions will spawn
  • First quarter -- Eggs are buried in the sand and develop
  • Between first quarter and New moon -- Eggs continue to develop
  • New moon -- Eggs are released from the sand and will hatch
  • Between new moon and Third quarter -- Grunions come ashore to spawn
  • Between Third quarter and Full moon -- Eggs continue to develop
This sort of highly tuned synchrony between a species and the moon is not unique to the animal kingdom (fiddler crabs are also highly attuned to lunar phases), but the degree of evolution in the case of the grunion is fabulous. Physiologically, how the grunion times its spawning behaviour to the lunar phase is uncertain, but it has been hypothesized that they are either able to detect minor changes in the light intensity of the moon or can detect the subtle pressure changes associated with the changing lunar phases.

Conservation Status and Relationship with Humans

Grunions can be 'fished', provided that the 'angler' has a valid fishing license from the state of California. During April and May, the grunion season is closed to fishermen in order to protect the species from over-exploitation. During the rest of the season, anglers may take as many as they wish (there is no bag limit), but all capture must be done by hand rather than by net. If you are interested in either fishing this species or in simply viewing this remarkable reproductive event, try to find uncrowded beaches. Consult the web or the newspaper for a tidal and lunar schedule, and try to bring a thermos of coffee with you, as the run can happen quite late in the evening.

The species appears to be in good shape in its native range, and it is not listed either by CITES or the US endangered species act as being threatened or endangered. However, any species with such a restricted range and such a highly evolved reproductive cycle may surely be affected by the changes in global water currents that are likely to accompany global warming.

Taxonomy

Class: Actinopterygii
Order: Atheriniformes
Family: Atherinidae
Genus, species: Leuresthes tenuis

*The gulf grunion, Leuresthes sardina, is a close relative to the California grunion, and is nearly identical in all respects except that it is found in the Gulf of California, and spawns both day and night.


Written with the help of http://www.dfg.ca.gov/mrd/grnindx3.html

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